1. Field of the Invention
This invention relates to a neck screw for an intramedullary nail.
2. Description of Prior Art
U.S. Pat. Nos. 5,176,681 and 5,454,813, the teachings of which are incorporated herein by reference, and also EP 0 257 118, have made known an osteosynthesis aid which consists of a locking nail and a neck screw. The locking nail is driven into the femur from the proximal end. In the proximal portion, the locking nail has an inclined bore the axis of which is directed approximately to the axis of the collum femoris. A neck screw is passed through this bore. It has a threaded portion which is self-tapping for example, and is propelled up into the head of the femur. From the proximal end of the nail, a fixing or set screw is inserted into a bore of the locking nail which, with its inner end, interacts with circumferentially spaced, axially parallel grooves in the shank of the neck screw to secure the neck screw against rotation while allowing it to slide in an axial direction. Such an osteosynthesis aid preponderantly serves for taking care of trochanteral and subtrochanteral fractures, but also for taking care of fractures of the collum femoris or fractures in the head region.
Also, compression may be exercised by means of a compression screw which is screwed into the distal portion of the neck screw and which interacts with the locking nail. This application case occurs where there are fractures in the head and neck areas.
In order that the locking nail and the neck screw may be mounted a target instrument kit is required to locate the bores in the locking nail from outside. Most of them include an instrument which is placed at the proximal end of the locking nail, and which has a strap which extends in a spaced relationship parallel to the femur when connected to the locking nail.
When the osteosynthesis aid described is used complications may occur in the area of the neck screw in taking care of pertrochanteral femur fractures having short head-and-neck fragments or in case of significant osteoporosis. The neck screw may break out when the bone substance is reduced or there is a femoral head malposition. In addition, there is a risk of secondary rotation of the head-and-neck fragment when the neck screw is in an eccentric position. Further, it is known to use a blade in lieu of a neck screw, which is passed through an opening in the locking nail. See for example, U.S. Pat. No. 3,433,220. Such a blade is more favorable with respect to anti-rotational stability. However, inserting the blade is far more problematic. Moreover, the compression of the fracture cannot be achieved by means of a blade.
It is an object of the invention to create a neck screw wherein the surface subjected to load is enlarged in the threaded portion of the neck screw.
The inventive neck screw has axially parallel grooves formed on opposite outer sides extending from the distal end, out of which at least one extends into the threaded portion. Furthermore, a fork-like blade is provided the legs of which are accommodated by the grooves. The cross-section of the blade legs preferably corresponds to that of the grooves, a slight clearance being left in between, however. The blade legs are interconnected at the distal end via an appropriate connecting portion which, in turn, is connected to the distal end of the neck screw in an appropriate way. Preferably, the connecting portion is annularly cylindrical and its outer diameter is approximately equal to the outer diameter of the shank of the neck screw. It may be attached by means of a screw which is threaded into a female thread at the distal end of the neck screw. Preferably, the annularly cylindrical portion, in turn, has a female thread for connection to an appropriate drive-in instrument.
At least one femur neck blade/groove extends into the threaded portion of the neck screw or even slightly beyond same. Preferably, however, two blade legs are used each approximately equal in length such that both of them will enlarge the surface area subjected to load in the threaded portion of the neck screw. This results in an efficient safety against any secondary head-and-neck rotation. The inventive neck screw is particularly advantageous when used in significant osteoporosis or an eccentric position of a coxa screw.
The grooves or blade legs are preferably designed in such a way that the outer surfaces of blade legs are position approximately at the level of the shank outer surface in the shank area of the neck screw. In another aspect of the invention, the grooves are adapted to extend in a more planar way or have a more shallow depth in the area of the threaded portion, a ramp being adapted to be provided between the groove sections which are different in depth. As a result, the blade legs will be spread apart and, hence, will be anchored more efficiently.
In another aspect of the invention, the ends of blade legs have a chamfer at their outside. This makes it easier to drive them in. In another aspect of the invention, the ends of blade legs are chamfered at their inside, which makes it easier to insert them in the grooves. This aim is also served by option chamfers in the groove walls in the entry area of blades as well as at the bottom of the groove.
The inventive neck screw is implanted in a usual manner. After being screwed in, it is fixed in the sense of rotation as is the case for the osteosynthesis aid described in the beginning, using a fixing screw and axially parallel grooves, for example, at the outside of the screw shank. A compression of the fracture may be effect if needed. Afterwards, the blade will be driven in, which may be watched via an image converter. When the blade legs should extend in a way that they protrude beyond the proximal end of the neck screw this may be seen on the image converter. After the driving member is removed the blade may be secured against any slide-out by means of a screw which is turned into the above-described female thread at the proximal end of the neck screw.
The inventive neck screw is fully compatible with the conventional, above-described osteosynthesis aid. The conventional target instrument kit may be used. The dynamic way of supporting the neck screw may be maintained. It is easy to handle the inventive neck screw. Explanation does not pose problems either. As an alternative, however, the inventive neck screw may even be inserted without an arresting blade.
The sole difference that proves to exist as compared to the conventional neck screws which are described above is that only two sliding and fixing grooves can be formed in the screw shank because blade-receiving grooves are out of question for the purpose here.